This invention relates to a valve control system for internal combustion engines which are capable of changing the valve lift characteristic of inlet and/or exhaust valves, and more particularly to a valve control system of this kind for automotive vehicles equipped with driving wheel slip control systems.
There is shown in FIG. 8 the general relationship between the engine rotational speed and the engine output torque in an internal combustion engine which is capable of switching the valve lift characteristic of inlet and/or exhaust valves between a low speed valve lift characteristic suitable for a lower engine rotational speed region and a high speed valve lift characteristic suitable for a higher engine rotational speed region. (Throughout the specification and claims, the term "valve lift characteristic" represents a valve opening period of an inlet and/or exhaust valve and/or an amount of lift of the inlet and/or exhaust valve.) In the lower engine rotational speed region, larger engine output torque can be obtained when the low speed valve lift characteristic is selected, while in the higher engine rotational speed region, larger engine output torque can be obtained when the high speed valve lift characteristic is selected. Normally, the valve lift characteristic which enables to obtain larger engine output torque is selected depending upon engine operating conditions (mainly upon the engine rotational speed). Therefore, if the valve lift characteristic is changed to one which is opposite to a normally-selected valve lift characteristic, the engine output torque can be decreased.
This feature of selection of the valve timing is utilized, e.g. in Japanese Patent Application No. 63-330938 filed by the present assignee. This application proposes a valve control system in which the degree of slip of driving wheels is reduced by changing the valve lift characteristic to one which enables to obtain smaller engine output torque (traction control).
The above proposed valve control system is effective in suppressing excessive slip of the driving wheels, since the engine output can be effectively decreased by fuel cut when the excessive slip state of the driving wheels is detected. However, according to the above valve control system, when the excessive slip state of the driving wheels is eliminated by the traction control, the valve lift characteristic is restored to the normal one. On the other hand, since the engine output is reduced by the traction control to cause a decrease in the engine rotational speed, the valve lift characteristic is changed over at a predetermined engine rotational speed. This can increase the frequency of changeover of the valve lift characteristic. More specifically, if the engine output is decreased while an excessive slip state of the driving wheels is detected, the engine operating condition changes, in particular, the engine rotational speed decreases, so that the engine, if it is in a higher engine rotational speed region, undergoes transition to a lower engine rotational speed region whereby the valve lift characteristic is changed over. Since the excessive slip state of the driving wheel(s) is being detected, the valve lift characteristic is changed from one enabling to obtain a larger engine output in the lower engine rotational speed region to one enabling to obtain a larger engine output in the higher engine rotational speed region, i.e. in a manner opposite to normal changeover of the valve lift characteristic carried out when the vehicle is not under the traction control. Thereafter, when the excessive slip of the driving wheels is eliminated, the valve lift characteristic is changed to a normal one enabling to obtain a larger engine output in the lower engine rotational speed region again, so that the engine output increases to increase the engine rotational speed, which results in changeover of the valve lift characteristic to one enabling to obtain a larger engine output in the higher engine rotational speed region. If an excessive slip state of the driving wheels is detected again when the engine is in the higher engine rotational speed region, the above process of the valve lift characteristic changeover control is repeated. Therefore, the above valve control system suffers from the problem that the valve lift characteristic is frequently changed to shorten the life of the valve lift characteristic-changeover mechanism.
Further, if the valve timing is restored to the normal one immediately after an excessive slip of the driving wheel has been eliminated, there is a possibility that the engine output torque suddenly increases to cause excessive slip of the driving wheels again, depending on road surface conditions, to necessitate changeover of the valve lift characteristic to an opposite one enabling to obtain a smaller engine output again. Thus, the changeover of the valve characteristic can also be repeated frequently to shorten the life of the valve lift characteristic-changeover mechanism.
Still further, if the valve lift characteristic is restored to a normal one enabling to obtain a larger engine output after elimination of an excessive slip state of the driving wheels, the engine output torque suddenly increases to thereby degrade the driveability and even lead to occurrence of excessive slip of the driving wheels immediately after restoration of the normal valve lift characteristic.